In certain applications, the integrity of a part weldment is of critical importance to assure against future service failures. One such example is a nuclear fuel rod which is comprised of a column of fuel pellets contained in an elongated cladding tube formed of a zirconium alloy. End plugs, also of a zirconium alloy, are welded to the open ends of the tube to seal the pellet column therein. These end plug welds must meet exacting standards of integrity it they are to withstand the high internal pressures developed over the fuel rod life. Heretofore, the head end plug weld has been effected as a bead weld, and thus visual inspection was adequate to verify the presence of sufficient weld cross section to withstand the high internal pressures. Random end plug weld samples were examined using X-ray radiography at several viewing angles. The resulting radiographs had to be visually examined, and porosities were detectable and measurable only if located in the several view planes. Porosity resolution was limited to pores in excess of a five mil diameter. Other end plug weld samples were destructively examined by metallographic sectioning mainly for the purpose of qualifying the welding process and detecting process excursion.
Recent improvements in bundle spacer designs require that the head end plug weld be a flush weld. Visual inspection of flush welds is not an adequate quality assurance test of weld integrity. That is, absent the weld bead, visual inspection can not determine that the weld is continuous and of a cross sectional area at least equal to that of the cladding tube. Consequently, non-destructive examination of the weld internal characteristics is necessary to determine if a flush weld meets the requisite quality assurance standards for integrity. In view of the criticality of this end plug weld, each and every one must be examined.
In the commonly assigned, copending application entitled "Apparatus for Assembling and Welding End Plugs to Nuclear Fuel Cladding Tubes and Inspecting the End Plug Welds on an Automated Basis (524,415), there is disclosed and claimed apparatus for welding a head end plug to the open ends of a succession of cladding tubes and then ultrasonically inspecting each end plug weld on an automated, production line basis. To expedite production, this ultrasonic inspection must be performed in timely fashion consistent with the time required to mate an end plug with the tube open end and weld them together. This inspection must include measuring the tube wall thickness adjacent the weld, detecting the presence and size of any flaws in the weld, determining the minimum net cross sectional area of the weld, and the extent of weld penetration into the tube open end. These determinations must then be tested against strict quality assurance standards to decide whether the end plug weld is acceptable or rejectable. All of this should be accomplished in a short period of time, e.g., less than thirty seconds, without human intervention in order that quality assurance inspection not pose a production bottleneck.
It is accordingly an object of the present invention to provide an improved method for non-destructively examining the characteristics of weldments.
A further object is to provide a method of the above-character for non-destructively examining the dimensional and internal characteristics of weldments.
An additional object is to provide a method of the above-character, wherein the ultrasonic echos are converted to inspection data indicative of the weldment characteristics for comparison against quality assurance standards pursuant to determining on an automated basis whether a weldment is acceptable or rejectable.
A still further object is to provide a method of the above-character for non-destructively examining the integrity of the weld uniting an end plug to an open end of a nuclear fuel cladding tube on automated production line basis.
Yet another object is to provide ultrasonic inspection apparatus for practicing the method of the above-noted character.
Other objects of the invention will in part be obvious an in part appear hereinafter.